Pace

Runners World has a great pace calculator that uses a simple formula:
http://runnersworld.com/cda/trainingcalculator
I was surprised to find out that with my current pace (05/01/10), which - if extrapolated 1:1 - should allow be a marathon time under 4 hours, is really not going to hold up all that well over 26.2 miles. It may be discouraging for some, but for me this this is a prevention tool for over-training. At this point, when I have suffered through a week of terrible runs, it's like a sign saying - don't worry about the time. Remember - just finishing will be HUGE.

Here's what I got. I haven't ran any races yet, however, for example sake, let's say that I can run 6 miles at 9m/mi (54 minutes):
     Marathon time = 4:17
     Long run = 10:54 - 12:15 m/mi - this is WAY slower than what I run now.
     Tempo run = 9:07 m/mi


Now when I input my target marathon time of 4 hours I get suggested training paces:
     Tempo run = 8:34
     Long run = 10:15 - 11:33

I am not completely comprehending the concept of a slow long run at this point. I understand that a shorter run can be faster than a long run. However, if I am training for a marathon, shouldn't I train to run the "long run" - ie - main prep for marathon - at my target marathon pace??

PS - how freaking cool is that PaceTat???
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An AMAZING article by competitiverunner.com almost resolves my dilemma... almost. Well, at least it convinces me (and that's a feat!) that I should in fact be running my long runs slower "to avoid overtraining" while "still reaping all of the benefits of a long run"; the article makes sure to point out that "race pace runs are important". "The trick for any athlete is to find the balance between long easy runs and faster marathon paced runs."
And.. Pete Pfitzinger has the answer!

Long runs at marathon race pace prepare you most directly for the demands of the marathon. The principle of specificity of training states that the most effective way to prepare for an event is to simulate that event as closely as possible in training. The closest way to simulate a marathon, of course, is to run 26.2 miles at marathon pace. Unfortunately, long runs at marathon pace are very hard on the body. If you run too far at marathon pace, the required recovery time will negate the benefits of the effort. Similarly, if you do long runs at marathon pace too often, you will greatly increase your likelihood of self-destructing through injury or overtraining.

Links
http://www.runnersworld.com/article/0,7120,s6-238-520--13101-0,00.html# 
http://www.runnersworld.com/article/0,7120,s6-238-244-255-11932-0,00.html#
http://pfitzinger.com/labreport.shtml
http://runningtimes.com/Article.aspx?ArticleID=4432&PageNum=5 
http://www.runnersworld.com/article/0,7120,s6-238-244--6946-3-3X5X7-4,00.html#
http://www.mcmillanrunning.com/training2.htm 
http://www.runningmania.com/resources/programs/index.php
http://homepage.mac.com/psminkey/running/page6/page6.html